The proneural gene amos promotes multiple dendritic neuron formation in the Drosophila peripheral nervous system

A wound-induced differentiation trajectory for neurons

Animals capable of whole-body regeneration can replace any missing cell type and regenerate fully functional new organs, including new brains, de novo. The regeneration of a new brain requires the formation of diverse neural cell types and their assembly into an organized structure with correctly wired circuits. Recent work in various regenerative animals has revealed transcriptional programs required for the differentiation of distinct neural subpopulations, however, how these transcriptional programs are initiated in response to injury remains unknown. Here, we focused on the highly regenerative acoel worm, Hofstenia miamia, to study wound-induced transcriptional regulatory events that lead to the production of neurons and subsequently a functional brain. Footprinting analysis using chromatin accessibility data on a chromosome-scale genome assembly revealed that binding sites for the Nuclear Factor Y (NFY) transcription factor complex were significantly bound during regeneration, showing a dynamic increase in binding within one hour upon amputation specifically in tail fragments, which will regenerate a new brain. Strikingly, NFY targets were highly enriched for genes with neuronal function. Single-cell transcriptome analysis combined with functional studies identified soxC+ stem cells as a putative progenitor population for multiple neural subtypes. Further, we found that wound-induced soxC expression is likely under direct transcriptional control by NFY, uncovering a mechanism for the initiation of a neural differentiation pathway by early wound-induced binding of a transcriptional regulator.

A wound-induced differentiation trajectory for neurons

Animals capable of whole-body regeneration can replace any missing cell type and regenerate fully-functional new organs, de novo . The regeneration of a new brain requires the formation of diverse neuronal cell types and their assembly into an organized structure and correctly-wired circuits. Recent work in various regenerative animals has revealed transcriptional programs required for the differentiation of distinct neuronal subpopulations, however how these transcriptional programs are initiated upon amputation remains unknown. Here, we focused on the highly regenerative acoel worm, Hofstenia miamia , to study wound-induced transcriptional regulatory events that lead to the production of neurons. Footprinting analysis using chromatin accessibility data on an improved genome assembly revealed that binding sites for the NFY transcription factor complex were significantly bound during regeneration, showing a dynamic increase in binding within one hour upon amputation specifically in tail fragments, which will regenerate a new brain. Strikingly, NFY targets were highly enriched for genes with neuronal functional. Single-cell transcriptome analysis combined with functional studies identified sox4 + stem cells as the likely progenitor population for multiple neuronal subtypes. Further, we found that wound-induced sox4 expression is likely under direct transcriptional control by NFY, uncovering a mechanism for how early wound-induced binding of a transcriptional regulator results in the initiation of a neuronal differentiation pathway.

Highlights

A new chromosome-scale assembly for Hofstenia enables comprehensive analysis of transcription factor binding during regeneration NFY motifs become dynamically bound by 1hpa in regenerating tail fragments, particularly in the loci of neural genes A sox4 + neural-specialized stem cell is identified using scRNA-seq sox4 is wound-induced and required for differentiation of multiple neural cell types NFY regulates wound-induced expression of sox4 during regeneration.

ASC proneural factors are necessary for chromatin remodeling during neuroectodermal to neuroblast fate transition to ensure the timely initiation of the neural stem cell program

Background

In both Drosophila and mammals, the achaete-scute (ASC/ASCL) proneural bHLH transcription factors are expressed in the developing central and peripheral nervous systems, where they function during specification and maintenance of the neural stem cells in opposition to Notch signaling. In addition to their role in nervous system development, ASC transcription factors are oncogenic and exhibit chromatin reprogramming activity; however, the impact of ASC on chromatin dynamics during neural stem cell generation remains elusive. Here, we investigate the chromatin changes accompanying neural commitment using an integrative genetics and genomics methodology.

Results

We found that ASC factors bind equally strongly to two distinct classes of cis-regulatory elements: open regions remodeled earlier during maternal to zygotic transition by Zelda and less accessible, Zelda-independent regions. Both classes of cis-elements exhibit enhanced chromatin accessibility during neural specification and correlate with transcriptional regulation of genes involved in a variety of biological processes necessary for neuroblast function/homeostasis. We identified an ASC-Notch regulated TF network that includes likely prime regulators of neuroblast function. Using a cohort of ASC target genes, we report that ASC null neuroblasts are defectively specified, remaining initially stalled, unable to divide, and lacking expression of many proneural targets. When mutant neuroblasts eventually start proliferating, they produce compromised progeny. Reporter lines driven by proneural-bound enhancers display ASC dependency, suggesting that the partial neuroblast identity seen in the absence of ASC genes is likely driven by other, proneural-independent, cis-elements. Neuroblast impairment and the late differentiation defects of ASC mutants are corrected by ectodermal induction of individual ASC genes but not by individual members of the TF network downstream of ASC. However, in wild-type embryos, the induction of individual members of this network induces CNS hyperplasia, suggesting that they synergize with the activating function of ASC to consolidate the chromatin dynamics that promote neural specification.

Conclusions

We demonstrate that ASC proneural transcription factors are indispensable for the timely initiation of the neural stem cell program at the chromatin level by regulating a large number of enhancers in the vicinity of neural genes. This early chromatin remodeling is crucial for both neuroblast homeostasis as well as future progeny fidelity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01300-8.

Function of Proneural Genes Ascl1 and Asense in Neurogenesis: How Similar Are They?

Proneural genes were initially identified in Drosophila, where pioneer work on these important regulators of neural development was performed, and from which the term proneural function was coined. Subsequently, their counterparts in vertebrates were identified, and their function in neural development extensively characterized. The function of proneural transcription factors in flies and vertebrates is, however, very distinct. In flies, proneural genes play an early role in neural induction, by endowing neural competence to ectodermal cells. In contrast, vertebrate proneural genes are expressed only after neural specification, in neural stem and progenitor cells, where they play key regulatory functions in quiescence, proliferation, and neuronal differentiation. An exception to this scenario is the Drosophila proneural gene asense, which has a late onset of expression in neural stem cells of the developing embryo and larvae, similar to its vertebrate counterparts. Although the role of Asense remains poorly investigated, its expression pattern is suggestive of functions more in line with those of vertebrate proneural genes. Here, we revise our current understanding of the multiple activities of Asense and of its closest vertebrate homologue Ascl1 in neural stem/progenitor cell biology, and discuss possible parallels between the two transcription factors in neurogenesis regulation.

Single-cell visualization of mir-9a and Senseless co-expression during Drosophila melanogaster embryonic and larval peripheral nervous system development

The Drosophila melanogaster peripheral nervous system (PNS) comprises the sensory organs that allow the fly to detect environmental factors such as temperature and pressure. PNS development is a highly specified process where each sensilla originates from a single sensory organ precursor (SOP) cell. One of the major genetic orchestrators of PNS development is Senseless, which encodes a zinc finger transcription factor (Sens). Sens is both necessary and sufficient for SOP differentiation. Senseless expression and SOP number are regulated by the microRNA miR-9a. However, the reciprocal dynamics of Senseless and miR-9a are still obscure. By coupling single-molecule FISH with immunofluorescence, we are able to visualize transcription of the mir-9a locus and expression of Sens simultaneously. During embryogenesis, we show that the expression of mir-9a in SOP cells is rapidly lost as Senseless expression increases. However, this mutually exclusive expression pattern is not observed in the third instar imaginal wing disc, where some Senseless-expressing cells show active sites of mir-9a transcription. These data challenge and extend previous models of Senseless regulation and show complex co-expression dynamics between mir-9a and Senseless. The differences in this dynamic relationship between embryonic and larval PNS development suggest a possible switch in miR-9a function. Our work brings single-cell resolution to the understanding of dynamic regulation of PNS development by Senseless and miR-9a.

Homeostatic maintenance and age-related functional decline in the Drosophila ear

Age-related hearing loss (ARHL) is a threat to future human wellbeing. Multiple factors contributing to the terminal auditory decline have been identified; but a unified understanding of ARHL - or the homeostatic maintenance of hearing before its breakdown - is missing. We here present an in-depth analysis of homeostasis and ageing in the antennal ears of the fruit fly Drosophila melanogaster. We show that Drosophila, just like humans, display ARHL. By focusing on the phase of dynamic stability prior to the eventual hearing loss we discovered a set of evolutionarily conserved homeostasis genes. The transcription factors Onecut (closest human orthologues: ONECUT2, ONECUT3), Optix (SIX3, SIX6), Worniu (SNAI2) and Amos (ATOH1, ATOH7, ATOH8, NEUROD1) emerged as key regulators, acting upstream of core components of the fly’s molecular machinery for auditory transduction and amplification. Adult-specific manipulation of homeostatic regulators in the fly’s auditory neurons accelerated - or protected against - ARHL.

Brn3/POU-IV-type POU homeobox genes-Paradigmatic regulators of neuronal identity across phylogeny

One approach to understand the construction of complex systems is to investigate whether there are simple design principles that are commonly used in building such a system. In the context of nervous system development, one may ask whether the generation of its highly diverse sets of constituents, that is, distinct neuronal cell types, relies on genetic mechanisms that share specific common features. Specifically, are there common patterns in the function of regulatory genes across different neuron types and are those regulatory mechanisms not only used in different parts of one nervous system, but are they conserved across animal phylogeny? We address these questions here by focusing on one specific, highly conserved and well-studied regulatory factor, the POU homeodomain transcription factor UNC-86. Work over the last 30 years has revealed a common and paradigmatic theme of unc-86 function throughout most of the neuron types in which Caenorhabditis elegans unc-86 is expressed. Apart from its role in preventing lineage reiterations during development, UNC-86 operates in combination with distinct partner proteins to initiate and maintain terminal differentiation programs, by coregulating a vast array of functionally distinct identity determinants of specific neuron types. Mouse orthologs of unc-86, the Brn3 genes, have been shown to fulfill a similar function in initiating and maintaining neuronal identity in specific parts of the mouse brain and similar functions appear to be carried out by the sole Drosophila ortholog, Acj6. The terminal selector function of UNC-86 in many different neuron types provides a paradigm for neuronal identity regulation across phylogeny. This article is categorized under: Gene Expression and Transcriptional Hierarchies > Regulatory Mechanisms Invertebrate Organogenesis > Worms Nervous System Development > Vertebrates: Regional Development.

The Drosophila fussel gene is required for bitter gustatory neuron differentiation acting within an Rpd3 dependent chromatin modifying complex

Members of the Ski/Sno protein family are classified as proto-oncogenes and act as negative regulators of the TGF-ß/BMP-pathways in vertebrates and invertebrates. A newly identified member of this protein family is fussel (fuss), the Drosophila homologue of the human functional Smad suppressing elements (fussel-15 and fussel-18). We and others have shown that Fuss interacts with SMAD4 and that overexpression leads to a strong inhibition of Dpp signaling. However, to be able to characterize the endogenous Fuss function in Drosophila melanogaster, we have generated a number of state of the art tools including anti-Fuss antibodies, specific fuss-Gal4 lines and fuss mutant fly lines via the CRISPR/Cas9 system. Fuss is a predominantly nuclear, postmitotic protein, mainly expressed in interneurons and fuss mutants are fully viable without any obvious developmental phenotype. To identify potential target genes or cells affected in fuss mutants, we conducted targeted DamID experiments in adult flies, which revealed the function of fuss in bitter gustatory neurons. We fully characterized fuss expression in the adult proboscis and by using food choice assays we were able to show that fuss mutants display defects in detecting bitter compounds. This correlated with a reduction of gustatory receptor gene expression (Gr33a, Gr66a, Gr93a) providing a molecular link to the behavioral phenotype. In addition, Fuss interacts with Rpd3, and downregulation of rpd3 in gustatory neurons phenocopies the loss of Fuss expression. Surprisingly, there is no colocalization of Fuss with phosphorylated Mad in the larval central nervous system, excluding a direct involvement of Fuss in Dpp/BMP signaling. Here we provide a first and exciting link of Fuss function in gustatory bitter neurons. Although gustatory receptors have been well characterized, little is known regarding the differentiation and maturation of gustatory neurons. This work therefore reveals Fuss as a pivotal element for the proper differentiation of bitter gustatory neurons acting within a chromatin modifying complex.

Genomic Signals of Adaptation towards Mutualism and Sociality in Two Ambrosia Beetle Complexes

Mutualistic symbiosis and eusociality have developed through gradual evolutionary processes at different times in specific lineages. Like some species of termites and ants, ambrosia beetles have independently evolved a mutualistic nutritional symbiosis with fungi, which has been associated with the evolution of complex social behaviors in some members of this group. We sequenced the transcriptomes of two ambrosia complexes (Euwallacea sp. near fornicatus⁻Fusarium euwallaceae and Xyleborus glabratus⁻Raffaelea lauricola) to find evolutionary signatures associated with mutualism and behavior evolution. We identified signatures of positive selection in genes related to nutrient homeostasis; regulation of gene expression; development and function of the nervous system, which may be involved in diet specialization; behavioral changes; and social evolution in this lineage. Finally, we found convergent changes in evolutionary rates of proteins across lineages with phylogenetically independent origins of sociality and mutualism, suggesting a constrained evolution of conserved genes in social species, and an evolutionary rate acceleration related to changes in selective pressures in mutualistic lineages.

Stretching the imagination beyond muscle spindles - stretch-sensitive mechanisms in arthropods

Much attention has been given to mammalian muscle spindles and their role in stretch‐mediated muscle proprioception. Recent studies, particularly, have sought to determine the molecular mediators of stretch‐evoked mechanotransduction, which these endings rely upon for functionality. Nonetheless, much about these endings remains unknown. Opportunities may be presented from consideration of extensive parallel research in stretch receptor mechanisms in arthropods. Such systems may provide a useful source of additional data and powerful tools for dissecting the complex systems of stretch transduction apparatus. At the least, such systems provide tractable exemplars of how organisms solve the problem of converting stretch stimuli to electrical output. Potentially, they may even provide molecular mechanisms and candidate molecular mediators of direct relevance to mammalian muscle spindles. Here we provide a brief overview of research on arthropod stretch receptors.

Salvador-Warts-Hippo pathway in a developmental checkpoint monitoring helix-loop-helix proteins

The E proteins and Id proteins are, respectively, the positive and negative heterodimer partners for the basic-helix-loop-helix protein family and as such contribute to a remarkably large number of cell-fate decisions. E proteins and Id proteins also function to inhibit or promote cell proliferation and cancer. Using a genetic modifier screen in Drosophila, we show that the Id protein Extramacrochaetae enables growth by suppressing activation of the Salvador-Warts-Hippo pathway of tumor suppressors, activation that requires transcriptional activation of the expanded gene by the E protein Daughterless. Daughterless protein binds to an intronic enhancer in the expanded gene, both activating the SWH pathway independently of the transmembrane protein Crumbs and bypassing the negative feedback regulation that targets the same expanded enhancer. Thus, the Salvador-Warts-Hippo pathway has a cell-autonomous function to prevent inappropriate differentiation due to transcription factor imbalance and monitors the intrinsic developmental status of progenitor cells, distinct from any responses to cell-cell interactions.

The Role of Atonal Factors in Mechanosensory Cell Specification and Function

Atonal genes are basic helix-loop-helix transcription factors that were first identified as regulating the formation of mechanoreceptors and photoreceptors in Drosophila. Isolation of vertebrate homologs of atonal genes has shown these transcription factors to play diverse roles in the development of neurons and their progenitors, gut epithelial cells, and mechanosensory cells in the inner ear and skin. In this article, we review the molecular function and regulation of atonal genes and their targets, with particular emphasis on the function of Atoh1 in the development, survival, and function of hair cells of the inner ear. We discuss cell-extrinsic signals that induce Atoh1 expression and the transcriptional networks that regulate its expression during development. Finally, we discuss recent work showing how identification of Atoh1 target genes in the cerebellum, spinal cord, and gut can be used to propose candidate Atoh1 targets in tissues such as the inner ear where cell numbers and biochemical material are limiting.

Development of the embryonic and larval peripheral nervous system of Drosophila

The peripheral nervous system (PNS) of embryonic and larval stage Drosophila consists of diverse types of sensory neurons positioned along the body wall. Sensory neurons, and associated end organs, show highly stereotyped locations and morphologies. Many powerful genetic tools for gene manipulation available in Drosophila make the PNS an advantageous system for elucidating basic principles of neural development. Studies of the Drosophila PNS have provided key insights into molecular mechanisms of cell fate specification, asymmetric cell division, and dendritic morphogenesis. A canonical lineage gives rise to sensory neurons and associated organs, and cells within this lineage are diversified through asymmetric cell divisions. Newly specified sensory neurons develop specific dendritic patterns, which are controlled by numerous factors including transcriptional regulators, interactions with neighboring neurons, and intracellular trafficking systems. In addition, sensory axons show modality specific terminations in the central nervous system, which are patterned by secreted ligands and their receptors expressed by sensory axons. Modality-specific axon projections are critical for coordinated larval behaviors. We review the molecular basis for PNS development and address some of the instances in which the mechanisms and molecules identified are conserved in vertebrate development.

Proneural bHLH genes in development and disease

Proneural genes encode evolutionarily conserved basic-helix-loop-helix transcription factors. In Drosophila, proneural genes are required and sufficient to confer a neural identity onto naïve ectodermal cells, inducing delamination and subsequent neuronal differentiation. In vertebrates, proneural genes are expressed in cells that already have a neural identity, but they are still required and sufficient to initiate neurogenesis. In all organisms, proneural genes control neurogenesis by regulating Notch-mediated lateral inhibition and initiating the expression of downstream differentiation genes. The general mode of proneural gene function has thus been elucidated. However, the regulatory mechanisms that spatially and temporally control proneural gene function are only beginning to be deciphered. Understanding how proneural gene function is regulated is essential, as aberrant proneural gene expression has recently been linked to a variety of human diseases-ranging from cancer to neuropsychiatric illnesses and diabetes. Recent insights into proneural gene function in development and disease are highlighted herein.

Genome-wide analysis of the bHLH gene family in planarians identifies factors required for adult neurogenesis and neuronal regeneration

In contrast to most well-studied model organisms, planarians have a remarkable ability to completely regenerate a functional nervous system from a pluripotent stem cell population. Thus, planarians provide a powerful model to identify genes required for adult neurogenesis in vivo. We analyzed the basic helix-loop-helix (bHLH) family of transcription factors, many of which are crucial for nervous system development and have been implicated in human diseases. However, their potential roles in adult neurogenesis or central nervous system (CNS) function are not well understood. We identified 44 planarian bHLH homologs, determined their patterns of expression in the animal and assessed their functions using RNAi. We found nine bHLHs expressed in stem cells and neurons that are required for CNS regeneration. Our analyses revealed that homologs of coe, hes (hesl-3) and sim label progenitors in intact planarians, and following amputation we observed an enrichment of coe(+) and sim(+) progenitors near the wound site. RNAi knockdown of coe, hesl-3 or sim led to defects in CNS regeneration, including failure of the cephalic ganglia to properly pattern and a loss of expression of distinct neuronal subtype markers. Together, these data indicate that coe, hesl-3 and sim label neural progenitor cells, which serve to generate new neurons in uninjured or regenerating animals. Our study demonstrates that this model will be useful to investigate how stem cells interpret and respond to genetic and environmental cues in the CNS and to examine the role of bHLH transcription factors in adult tissue regeneration.

Functional genomic analyses of two morphologically distinct classes of Drosophila sensory neurons: post-mitotic roles of transcription factors in dendritic patterning

Background

Neurons are one of the most structurally and functionally diverse cell types found in nature, owing in large part to their unique class specific dendritic architectures. Dendrites, being highly specialized in receiving and processing neuronal signals, play a key role in the formation of functional neural circuits. Hence, in order to understand the emergence and assembly of a complex nervous system, it is critical to understand the molecular mechanisms that direct class specific dendritogenesis.

Methodology/Principal Findings

We have used the Drosophila dendritic arborization (da) neurons to gain systems-level insight into dendritogenesis by a comparative study of the morphologically distinct Class-I (C-I) and Class-IV (C-IV) da neurons. We have used a combination of cell-type specific transcriptional expression profiling coupled to a targeted and systematic in vivo RNAi functional validation screen. Our comparative transcriptomic analyses have revealed a large number of differentially enriched/depleted gene-sets between C-I and C-IV neurons, including a broad range of molecular factors and biological processes such as proteolytic and metabolic pathways. Further, using this data, we have identified and validated the role of 37 transcription factors in regulating class specific dendrite development using in vivo class-specific RNAi knockdowns followed by rigorous and quantitative neurometric analysis.

Conclusions/Significance

This study reports the first global gene-expression profiles from purified Drosophila C-I and C-IV da neurons. We also report the first large-scale semi-automated reconstruction of over 4,900 da neurons, which were used to quantitatively validate the RNAi screen phenotypes. Overall, these analyses shed global and unbiased novel insights into the molecular differences that underlie the morphological diversity of distinct neuronal cell-types. Furthermore, our class-specific gene expression datasets should prove a valuable community resource in guiding further investigations designed to explore the molecular mechanisms underlying class specific neuronal patterning.

The Drosophila BTB domain protein Jim Lovell has roles in multiple larval and adult behaviors

Innate behaviors have their origins in the specification of neural fates during development. Within Drosophila, BTB (Bric-a-brac,Tramtrack, Broad) domain proteins such as Fruitless are known to play key roles in the neural differentiation underlying such responses. We previously identified a gene, which we have termed jim lovell (lov), encoding a BTB protein with a role in gravity responses. To understand more fully the behavioral roles of this gene we have investigated its function through several approaches. Transcript and protein expression patterns have been examined and behavioral phenotypes of new lov mutations have been characterized. Lov is a nuclear protein, suggesting a role as a transcriptional regulator, as for other BTB proteins. In late embryogenesis, Lov is expressed in many CNS and PNS neurons. An examination of the PNS expression indicates that lov functions in the late specification of several classes of sensory neurons. In particular, only two of the five abdominal lateral chordotonal neurons express Lov, predicting functional variation within this highly similar group. Surprisingly, Lov is also expressed very early in embryogenesis in ways that suggests roles in morphogenetic movements, amnioserosa function and head neurogenesis. The phenotypes of two new lov mutations that delete adjacent non-coding DNA regions are strikingly different suggesting removal of different regulatory elements. In lov⁴⁷, Lov expression is lost in many embryonic neurons including the two lateral chordotonal neurons. lov⁴⁷ mutant larvae show feeding and locomotor defects including spontaneous backward movement. Adult lov⁴⁷ males perform aberrant courtship behavior distinguished by courtship displays that are not directed at the female. lov⁴⁷ adults also show more defective negative gravitaxis than the previously isolated lov^91Y mutant. In contrast, lov⁶⁶ produces largely normal behavior but severe female sterility associated with ectopic lov expression in the ovary. We propose a negative regulatory role for the DNA deleted in lov⁶⁶.

SRY induced TCF21 genome-wide targets and cascade of bHLH factors during Sertoli cell differentiation and male sex determination in rats

Male sex determination is initiated through the testis-determining factor SRY that promotes Sertoli cell differentiation and subsequent gonadal development. The basic helix-loop-helix (bHLH) gene Tcf21 was identified as one of the direct downstream targets of SRY. The current study was designed to identify the downstream targets of TCF21 and the potential cascade of bHLH genes that promote Sertoli cell differentiation. A modified ChIP-Chip comparative hybridization analysis identified 121 direct downstream binding targets for TCF21. The gene networks and cellular pathways potentially regulated by these TCF21 targets were identified. One of the main bHLH targets for TCF21 was the bHLH gene scleraxis (Scx). An embryonic ovarian gonadal cell culture was used to examine the functional role of Sry, Tcf21, and Scx to promote an in vitro sex reversal and induction of Sertoli cell differentiation. SRY and TCF21 were found to induce the initial stages of Sertoli cell differentiation, whereas SCX was found to induce the later stages of Sertoli cell differentiation associated with pubertal development using transferrin gene expression as a marker. Therefore, a cascade of SRY followed by TCF21 followed by SCX appears to promote, in part, Sertoli cell fate determination and subsequent differentiation. The current observations help elucidate the initial molecular events involved in the induction of Sertoli cell differentiation and testis development.

Essential roles of Da transactivation domains in neurogenesis and in E(spl)-mediated repression

E proteins are a special class of basic helix-loop-helix (bHLH) proteins that heterodimerize with many bHLH activators to regulate developmental decisions, such as myogenesis and neurogenesis. Daughterless (Da) is the sole E protein in Drosophila and is ubiquitously expressed. We have characterized two transcription activation domains (TADs) in Da, called activation domain 1 (AD1) and loop-helix (LH), and have evaluated their roles in promoting peripheral neurogenesis. In this context, Da heterodimerizes with proneural proteins, such as Scute (Sc), which is dynamically expressed and also contributes a TAD. We found that either one of the Da TADs in the Da/Sc complex is sufficient to promote neurogenesis, whereas the Sc TAD is incapable of doing so. Besides its transcriptional activation role, the Da AD1 domain serves as an interaction platform for E(spl) proteins, bHLH-Orange family repressors which antagonize Da/Sc function. We show that the E(spl) Orange domain is needed for this interaction and strongly contributes to the antiproneural activity of E(spl) proteins. We present a mechanistic model on the interplay of these bHLH factors in the context of neural fate assignment.

A network of broadly expressed HLH genes regulates tissue-specific cell fates

Spatial and temporal expression of specific basic-helix-loop-helix (bHLH) transcription factors defines many types of cellular differentiation. We find that a distinct mechanism regulates the much broader expression of the heterodimer partners of these specific factors and impinges on differentiation. In Drosophila, a cross-interacting regulatory network links expression of the E protein Daughterless (Da), which heterodimerizes with bHLH proteins to activate them, with expression of the Id protein Extramacrochaetae (Emc), which antagonizes bHLH proteins. Coupled transcriptional feedback loops maintain the widespread Emc expression that restrains Da expression, opposing bHLH-dependent differentiation while enhancing growth and cell survival. Where extracellular signals repress emc, Da expression can increase. This defines regions of proneural ectoderm independently from the proneural bHLH genes. Similar regulation is found in multiple Drosophila tissues and in mammalian cells and therefore is likely to be a conserved general feature of developmental regulation by HLH proteins.

Identification of Bombyx atonal and functional comparison with the Drosophila atonal proneural factor in the developing fly eye

The proneural genes are fundamental regulators of neuronal development in all metazoans. A critical role of the fly proneural factor Atonal (Ato(Dm)) is to induce photoreceptor neuron formation in Drosophila, whereas its murine homolog, Atonal7(Mm) (aka Ath5) is essential for the development of the ganglion cells of the vertebrate eye. Here, we identify the Bombyx mori ato homolog (ato(Bm) ). In a pattern strikingly reminiscent of ato(Dm), the ato(Bm) mRNA is expressed as a stripe in the silkworm eye disc. Its DNA-binding and protein-protein interaction domain is highly homologous to the Ato(Dm) bHLH. Targeted expression of Ato(Bm) in the endogenous ato(Dm) pattern rescues the eyeless phenotype of the fly ato(1) mutant and its ectopic expression induces similar gain-of-function phenotypes as Ato(Dm). Rescue experiments with chimeric proteins show that the non-bHLH portion of Ato(Bm) (N-region) can effectively substitute for the corresponding region of the fly transcription factor, even though no apparent conservation can be found at the amino acid level. On the contrary, the highly similar bHLH domain of Ato(Bm) cannot similarly substitute for the corresponding region of Ato(Dm). Thus, the bHLH(Bm) domain requires the Ato(Bm) N-region to function effectively, whereas the bHLH(Dm) domain can operate well with either N-region. These findings suggest a role for the non-bHLH portion of Ato proteins in modulating the function of the bHLH domain in eye neurogenesis and implicate specific aa residues of the bHLH in this process.

A theory for the arrangement of sensory organs in Drosophila

We study the arrangements of recurved bristles on the anterior wing margin of wild-type and mutant Drosophila. The epidermal or neural fate of a proneural cell depends on the concentrations of proteins of the achaete-scute complex. At puparium formation, concentrations of proteins are nearly identical in all cells of the anterior wing and each cell has the potential for neural fate. In wild-type flies, the action of regulatory networks drives the initial state to one where a bristle grows out of every fifth cell. Recent experiments have shown that the frequency of recurved bristles can be made to change by adjusting the mean concentrations of the zinc-finger transcription factor Senseless and the micro-RNA miR-9a. Specifically, mutant flies with reduced levels of miR-9a exhibit ectopic bristles, and those with lower levels of both miR-9a and Senseless show regular organization of recurved bristles, but with a lower periodicity of 4. We argue that these characteristics can be explained assuming an underlying Turing-type bifurcation whereby a periodic pattern spontaneously emerges from a uniform background. However, bristle patterns occur in a discrete array of cells, and are not mediated by diffusion. We argue that intracellular actions of transmembrane proteins such as Delta and Notch can play a role of diffusion in destabilizing the homogeneous state. In contrast to diffusion, intercellular actions can be activating or inhibiting; further, there can be lateral cross-species interactions. We introduce a phenomenological model to study bristle arrangements and make several model-independent predictions that can be tested in experiments. In our theory, miRNA-9a is one of the components of the underlying network and has no special regulatory role. The loss of periodicity in its absence is due to the transfer of the system to a bistable state.

Drosophila muscleblind is involved in troponin T alternative splicing and apoptosis

Background

Muscleblind-like proteins (MBNL) have been involved in a developmental switch in the use of defined cassette exons. Such transition fails in the CTG repeat expansion disease myotonic dystrophy due, in part, to sequestration of MBNL proteins by CUG repeat RNA. Four protein isoforms (MblA-D) are coded by the unique Drosophila muscleblind gene.

Methodology/Principal Findings

We used evolutionary, genetic and cell culture approaches to study muscleblind (mbl) function in flies. The evolutionary study showed that the MblC protein isoform was readily conserved from nematods to Drosophila, which suggests that it performs the most ancestral muscleblind functions. Overexpression of MblC in the fly eye precursors led to an externally rough eye morphology. This phenotype was used in a genetic screen to identify five dominant suppressors and 13 dominant enhancers including Drosophila CUG-BP1 homolog aret, exon junction complex components tsunagi and Aly, and pro-apoptotic genes Traf1 and reaper. We further investigated Muscleblind implication in apoptosis and splicing regulation. We found missplicing of troponin T in muscleblind mutant pupae and confirmed Muscleblind ability to regulate mouse fast skeletal muscle Troponin T (TnnT3) minigene splicing in human HEK cells. MblC overexpression in the wing imaginal disc activated apoptosis in a spatially restricted manner. Bioinformatics analysis identified a conserved FKRP motif, weakly resembling a sumoylation target site, in the MblC-specific sequence. Site-directed mutagenesis of the motif revealed no change in activity of mutant MblC on TnnT3 minigene splicing or aberrant binding to CUG repeat RNA, but altered the ability of the protein to form perinuclear aggregates and enhanced cell death-inducing activity of MblC overexpression.

Conclusions/Significance

Taken together our genetic approach identify cellular processes influenced by Muscleblind function, whereas in vivo and cell culture experiments define Drosophila troponin T as a new Muscleblind target, reveal a potential involvement of MblC in programmed cell death and recognize the FKRP motif as a putative regulator of MblC function and/or subcellular location in the cell.

Evolutionary changes in sensory precursor formation in arthropods: embryonic development of leg sensilla in the spider Cupiennius salei

We describe here for the first time the development of mechanosensory organs in a chelicerate, the spider Cupiennius salei. It has been shown previously that the number of external sense organs increases with each moult. While stage 1 larvae do not have any external sensory structures, stage 2 larvae show a stereotyped pattern of touch sensitive 'tactile hairs' on their legs. We show that these mechanosensory organs develop during embryogenesis. In contrast to insects, groups of sensory precursors are recruited from the leg epithelium, rather than single sensory organ progenitors. The groups increase by proliferation, and neural cells delaminate from the cluster, which migrate away to occupy a position proximal to the accessory cells of the sense organ. In addition, we describe the development of putative internal sense organs, which do not differentiate until larval stage 2. We show by RNA interference that, similar to Drosophila, proneural genes are responsible for the formation and subtype identity of sensory organs. Furthermore, we demonstrate an additional function for proneural genes in the coordinated invagination and migration of neural cells during sensory organ formation in the spider.

Functional distinctness of closely related transcription factors: a comparison of the Atonal and Amos proneural factors

Using the well-characterised paradigm of Drosophila sensory nervous system development, we examine the functional distinctness of the Amos and Atonal (Ato) proneural transcription factors, which have different mutant phenotypes but share very high similarity in their signature bHLH domains. Using misexpression and mutant rescue assays, we show that Ato and Amos proteins have abundantly distinct intrinsic proneural capabilities in much of the ectoderm. The eye, however, is an exception: here both proteins share the capability to direct the R8 photoreceptor fate choice. Therefore, functional distinctness between these closely related transcription factors vary with developmental context, indicating different molecular mechanisms of specificity in different contexts. Consistent with this, the structural basis for their distinctness also varies depending upon the function in question. In previous studies of neural bHLH factors, specificity invariably mapped to the bHLH domain sequence. Similarly, and despite their high similarity, much of the Amos' specificity relative to Ato maps to Amos-specific residues in its bHLH domain. For Ato-specific functions, however, the Amos bHLH domain can substitute for that of Ato. Consequently, Ato's specificity relative to Amos requires the non-bHLH portion of the Ato protein. Ato provides a powerful precedence for a role of non-bHLH sequences in modulating bHLH functional specificity. This has implications for structural and functional comparisons of other closely related transcription factors, and for understanding the molecular basis of specificity.

Dorsal longitudinal stretch receptor of Drosophila melanogaster larva - fine structure and maturation

Insects possess two types of sensory neurons: ciliated type I sensory neurons that innervate external sensory organs and chordotonal organs, and type II sensory neurons that form a subepidermal plexus or innervate stretch receptors. Among stretch receptors, a dorsel longitudinal stretch receptor is highly conserved in insects, being found in all insect orders investigated. Here we describe the topology and anatomical structure of this receptor in the fruit fly embryo and larva using transmission electron microscopy and single cell staining for fluorescence microscopy. The receptor is composed of the dorsal bipolar dendrite neuron, which arises from an archetypal cell lineage, its sister glial cell and the peripheral glial cell accompanying the nerve. The neuron is situated among the muscles in the dorsal body wall on the intersegmental nerve. Its two dendrites stretch the length of the segment to the segmental folds. The neuron is wrapped by both glial cells and surrounded by a common basal lamina, which fans out at the dendritic tips to attach them to the epidermal cells at the segmental borders.

Role of proneural genes in the formation of the larval olfactory organ of Drosophila

In this paper, we address the role of proneural genes in the formation of the dorsal organ in the Drosophila larva. This organ is an intricate compound comprising the multineuronal dome-the exclusive larval olfactory organ-and a number of mostly gustatory sensilla. We first determine the numbers of neurons and of the different types of accessory cells in the dorsal organ. From these data, we conclude that the dorsal organ derives from 14 sensory organ precursor cells. Seven of them appear to give rise to the dome, which therefore may be composed of seven fused sensilla, whereas the other precursors produce the remaining sensilla of the dorsal organ. By a loss-of-function approach, we then analyze the role of atonal, amos, and the achaete-scute complex (AS-C), which in the adult are the exclusive proneural genes required for chemosensory organ specification. We show that atonal and amos are necessary and sufficient in a complementary way for four and three of the sensory organ precursors of the dome, respectively. AS-C, on the other hand, is implicated in specifying the non-olfactory sensilla, partially in cooperation with atonal and/or amos. Similar links for these proneural genes with olfactory and gustatory function have been established in the adult fly. However, such conserved gene function is not trivial, given that adult and larval chemosensory organs are anatomically very different and that the development of adult olfactory sensilla involves cell recruitment, which is unlikely to play a role in dome formation.

Multiple enhancers contribute to spatial but not temporal complexity in the expression of the proneural gene, amos

Background

The regulation of proneural gene expression is an important aspect of neurogenesis. In the study of the Drosophila proneural genes, scute and atonal, several themes have emerged that contribute to our understanding of the mechanism of neurogenesis. First, spatial complexity in proneural expression results from regulation by arrays of enhancer elements. Secondly, regulation of proneural gene expression occurs in distinct temporal phases, which tend to be under the control of separate enhancers. Thirdly, the later phase of proneural expression often relies on positive autoregulation. The control of these phases and the transition between them appear to be central to the mechanism of neurogenesis. We present the first investigation of the regulation of the proneural gene, amos.

Results

Amos protein expression has a complex pattern and shows temporally distinct phases, in common with previously characterised proneural genes. GFP reporter gene constructs were used to demonstrate that amos has an array of enhancer elements up- and downstream of the gene, which are required for different locations of amos expression. However, unlike other proneural genes, there is no evidence for separable enhancers for the different temporal phases of amos expression. Using mutant analysis and site-directed mutagenesis of potential Amos binding sites, we find no evidence for positive autoregulation as an important part of amos control during neurogenesis.

Conclusion

For amos, as for other proneural genes, a complex expression pattern results from the sum of a number of simpler sub-patterns driven by specific enhancers. There is, however, no apparent separation of enhancers for distinct temporal phases of expression, and this correlates with a lack of positive autoregulation. For scute and atonal, both these features are thought to be important in the mechanism of neurogenesis. Despite similarities in function and expression between the Drosophila proneural genes, amos is regulated in a fundamentally different way from scute and atonal.

MicroRNA-9a ensures the precise specification of sensory organ precursors in Drosophila

MicroRNAs (miRNAs) have been implicated in regulating various aspects of animal development, but their functions in neurogenesis are largely unknown. Here we report that loss of miR-9a function in the Drosophila peripheral nervous system leads to ectopic production of sensory organ precursors (SOPs), whereas overexpression of miR-9a results in a severe loss of SOPs. We further demonstrate a strong genetic interaction between miR-9a and senseless (sens) in controlling the formation of SOPs in the adult wing imaginal disc. Moreover, miR-9a suppresses Sens expression through its 3' untranslated region. miR-9a is expressed in epithelial cells, including those adjacent to SOPs within proneural clusters, suggesting that miR-9a normally inhibits neuronal fate in non-SOP cells by down-regulating Sens expression. These results indicate that miR-9a ensures the generation of the precise number of neuronal precursor cells during development.

An ancient transcriptional regulatory linkage

Changes in gene regulatory networks are a major engine for creating developmental novelty during evolution. Conversely, regulatory linkages that survive for very long evolutionary periods might be characteristic of ancient and abstract functions of fundamental utility to all metazoans. The proneural genes, which encode a distinctive family of basic helix-loop-helix (bHLH) transcriptional activators, act to promote neural cell fates in the ectoderm of diverse species. Here we report that these genes have been associated for at least 600-700 million years--since before the cnidarian/bilaterian divergence--with a high-affinity binding site for Hairy/Enhancer of split (Hes) repressor proteins. We suggest that the systematic identification of such ancient and conserved connections will be a powerful means of uncovering the primordial functions of transcription factors and signaling systems.

Genetics of graviperception in animals

Gravity is a constant stimulus for life on Earth and most organisms have evolved structures to sense gravitational force and incorporate its influence into their behavioral repertoire. Here we focus attention on animals and their diverse structures for perceiving and responding to the gravitational vector-one of the few static reference stimuli for any mobile organism. We discuss vertebrate, arthropod, and nematode models from the perspective of the role that genetics is playing in our understanding of graviperception in each system. We describe the key sensory structures in each class of organism and present what is known about the genetic control of development of these structures and the molecular signaling pathways operating in the mature organs. We also discuss the role of large genetic screens in identifying specific genes with roles in mechanosensation and graviperception.

The expression pattern of genes involved in early neurogenesis suggests distinct and conserved functions in the diplopod Glomeris marginata

We have shown recently that the expression and function of proneural genes is conserved in chelicerates and myriapods, although groups of neural precursors are specified in the ventral neuroectoderm of these arthropod groups, rather than single cells as in insects and crustaceans. We present additional evidence that the pattern of neurogenesis seen in chelicerates and in previously analyzed myriapod species is representative of both arthropod groups, by analysing the formation of neural precursors in the diplopod Archispirostreptus sp. This raises the question as to what extent the genetic network has been modified to result in different modes of neurogenesis in the arthropod group. To find out which components of the neural genetic network might account for the different mode of neural precursor formation in chelicerates and myriapods, we identified genes in the diplopod Glomeris marginata that are known to be involved in early neurogenesis in Drosophila and studied their expression pattern. In Drosophila, early neurogenesis is controlled by proneural genes that encode HLH transcription factors. These genes belong to two major subfamilies, the achaete-scute group and the atonal group. Different proneural proteins activate both a common neural programme and distinct neuronal subtype-specific target genes. We show that the expression pattern of homologs of the Drosophila proneural genes daughterless, atonal, and Sox B1 are partially conserved in Glomeris mariginata. While the expression of the pan-neural gene snail is conserved in the ventral neuroectoderm of G. marginata, we found an additional expression domain in the ventral midline. We conclude that, although the components of the genetic network involved in specification of neural precursors seem to be conserved in chelicerates, myriapods, and Drosophila, the function of some of the genes might have changed during evolution.

Senseless physically interacts with proneural proteins and functions as a transcriptional co-activator

The zinc-finger transcription factor Senseless is co-expressed with basic helix-loop-helix (bHLH) proneural proteins in Drosophila sensory organ precursors and is required for their normal development. High levels of Senseless synergize with bHLH proteins and upregulate target gene expression,whereas low levels of Senseless act as a repressor in vivo. However, the molecular mechanism for this dual role is unknown. Here, we show that Senseless binds bHLH proneural proteins via its core zinc fingers and is recruited by proneural proteins to their target enhancers to function as a co-activator. Some point mutations in the Senseless zinc-finger region abolish its DNA-binding ability but partially spare the ability of Senseless to synergize with proneural proteins and to induce sensory organ formation in vivo. Therefore, we propose that the structural basis for the switch between the repressor and co-activator functions of Senseless is the ability of its core zinc fingers to interact physically with both DNA and bHLH proneural proteins. As Senseless zinc fingers are ∼90% identical to the corresponding zinc fingers of its vertebrate homologue Gfi1, which is thought to cooperate with bHLH proteins in several contexts, the Senseless/bHLH interaction might be evolutionarily conserved.

Senseless and Daughterless confer neuronal identity to epithelial cells in the Drosophila wing margin

The basic helix-loop-helix (bHLH) proneural proteins Achaete and Scute cooperate with the class I bHLH protein Daughterless to specify the precursors of most sensory bristles in Drosophila. However, the mechanosensory bristles at the Drosophila wing margin have been reported to be unaffected by mutations that remove Achaete and Scute function. Indeed, the proneural gene(s) for these organs is not known. Here, we show that the zinc-finger transcription factor Senseless, together with Daughterless, plays the proneural role for the wing margin mechanosensory precursors, whereas Achaete and Scute are required for the survival of the mechanosensory neuron and support cells in these lineages. We provide evidence that Senseless and Daughterless physically interact and synergize in vivo and in transcription assays. Gain-of-function studies indicate that Senseless and Daughterless are sufficient to generate thoracic sensory organs (SOs) in the absence of achaete-scute gene complex function. However, analysis of senseless loss-of-function clones in the thorax implicates Senseless not in the primary SO precursor (pI) selection, but in the specification of pI progeny. Therefore, although Senseless and bHLH proneural proteins are employed during the development of all Drosophila bristles, they play fundamentally different roles in different subtypes of these organs. Our data indicate that transcription factors other than bHLH proteins can also perform the proneural function in the Drosophila peripheral nervous system.

Phosphorylation of Neurogenin2 specifies the migration properties and the dendritic morphology of pyramidal neurons in the neocortex

The molecular mechanisms specifying the dendritic morphology of different neuronal subtypes are poorly understood. Here we demonstrate that the bHLH transcription factor Neurogenin2 (Ngn2) is both necessary and sufficient for specifying the dendritic morphology of pyramidal neurons in vivo by specifying the polarity of its leading process during the initiation of radial migration. The ability of Ngn2 to promote a polarized leading process outgrowth requires the phosphorylation of a single tyrosine residue at position 241, an event that is neither involved in Ngn2 direct transactivation properties nor its proneural function. Interestingly, the migration defect observed in the Ngn2 knockout mouse and in progenitors expressing the Ngn2(Y241F) mutation can be rescued by inhibiting the activity of the small-GTPase RhoA in cortical progenitors. Our results demonstrate that Ngn2 coordinates the acquisition of the radial migration properties and the unipolar dendritic morphology characterizing pyramidal neurons through molecular mechanisms distinct from those mediating its proneural activity.

Charlatan, a Zn-finger transcription factor, establishes a novel level of regulation of the proneural achaete/scute genes of Drosophila

The proneural genes achaete (ac) and scute (sc) are necessary for the formation of the external sensory organs (SOs) of Drosophila. ac and sc are expressed in proneural clusters and impart their cells with neural potential. For this potential to be realized, and the SO precursor cell (SOP) to arise within a cluster, sufficient proneural protein must accumulate in the cluster. Here we describe a novel gene, charlatan (chn), which encodes a zinc finger transcription factor that facilitates this accumulation by forming a stimulatory loop with ac/sc. We find that loss of function of chn decreases the accumulation of Sc in proneural clusters and partially removes notum macrochaetae, while overexpression of chn enhances ac/sc expression and the formation of extra SOs. Moreover, chn is activated by ac/sc in proneural clusters. Chn apparently stimulates ac/sc by physically interacting with the proneural cluster-specific enhancers and increasing enhancer efficiency, thus acting as a stimulator of ac/sc expression in proneural clusters. chn is also required for the proper development of the embryonic peripheral nervous system, as its absence leads to loss of neurons and causes aberrant development of chordotonal organs.

The Drosophila ortholog of vertebrate TRPA1 regulates thermotaxis

Thermotaxis is important for animal survival, but the molecular identities of temperature sensors controlling this behavior have not been determined. We demonstrate dTRPA1, a heat-activated Transient Receptor Potential (TRP) family ion channel, is essential for thermotaxis in Drosophila. dTrpA1 knockdown eliminates avoidance of elevated temperatures along a thermal gradient. We observe dTRPA1 expression in cells without previously ascribed roles in thermosensation and implicate dTRPA1-expressing neurons in mediating thermotaxis. Our data suggest that thermotaxis relies upon neurons and molecules distinct from those required for high-temperature nociception. We propose dTRPA1 may control thermotaxis by sensing environmental temperature.

Control of central synaptic specificity in insect sensory neurons

Synaptic specificity is the culmination of several processes, beginning with the establishment of neuronal subtype identity, followed by navigation of the axon to the correct subdivision of neuropil, and finally, the cell-cell recognition of appropriate synaptic partners. In this review we summarize the work on sensory neurons in crickets, cockroaches, moths, and fruit flies that establishes some of the principles and molecular mechanisms involved in the control of synaptic specificity. The identity of a sensory neuron is controlled by combinatorial expression of transcription factors, the products of patterning and proneural genes. In the nervous system, sensory axon projections are anatomically segregated according to modality, stimulus quality, and cell-body position. A variety of cell-surface and intracellular signaling molecules are used to achieve this. Synaptic target recognition is also controlled by transcription factors such as Engrailed and may be, in part, mediated by cadherin-like molecules.

phyllopod is a target gene of proneural proteins in Drosophila external sensory organ development

Proneural basic helix-loop-helix (bHLH) proteins initiate neurogenesis in both vertebrates and invertebrates. The Drosophila Achaete (Ac) and Scute (Sc) proteins are among the first identified members of the large bHLH proneural protein family. phyllopod (phyl), encoding an ubiquitin ligase adaptor, is required for ac- and sc-dependent external sensory (ES) organ development. Expression of phyl is directly activated by Ac and Sc. Forced expression of phyl rescues ES organ formation in ac and sc double mutants. phyl and senseless, encoding a Zn-finger transcriptional factor, depend on each other in ES organ development. Our results provide the first example that bHLH proneural proteins promote neurogenesis through regulation of protein degradation.

Evolution of neural precursor selection: functional divergence of proneural proteins

How conserved pathways are differentially regulated to produce diverse outcomes is a fundamental question of developmental and evolutionary biology. The conserved process of neural precursor cell (NPC) selection by basic helix-loop-helix (bHLH) proneural transcription factors in the peripheral nervous system (PNS) by atonal related proteins (ARPs) presents an excellent model in which to address this issue. Proneural ARPs belong to two highly related groups: the ATONAL (ATO) group and the NEUROGENIN (NGN) group. We used a cross-species approach to demonstrate that the genetic and molecular mechanisms by which ATO proteins and NGN proteins select NPCs are different. Specifically, ATO group genes efficiently induce neurogenesis in Drosophila but very weakly in Xenopus, while the reverse is true for NGN group proteins. This divergence in proneural activity is encoded by three residues in the basic domain of ATO proteins. In NGN proteins, proneural capacity is encoded by the equivalent three residues in the basic domain and a novel motif in the second Helix (H2) domain. Differential interactions with different types of zinc (Zn)-finger proteins mediate the divergence of ATO and NGN activities: Senseless is required for ATO group activity, whereas MyT1 is required for NGN group function. These data suggest an evolutionary divergence in the mechanisms of NPC selection between protostomes and deuterostomes.

Senseless acts as a binary switch during sensory organ precursor selection

During sensory organ precursor (SOP) specification, a single cell is selected from a proneural cluster of cells. Here, we present evidence that Senseless (Sens), a zinc-finger transcription factor, plays an important role in this process. We show that Sens is directly activated by proneural proteins in the presumptive SOPs and a few cells surrounding the SOP in most tissues. In the cells that express low levels of Sens, it acts in a DNA-binding-dependent manner to repress transcription of proneural genes. In the presumptive SOPs that express high levels of Sens, it acts as a transcriptional activator and synergizes with proneural proteins. We therefore propose that Sens acts as a binary switch that is fundamental to SOP selection.

Senseless is required for pupal retinal development inDrosophila

Drosophila sensory organs are specified by a family of proneural genes which induce the expression of several common targets. One such target is senseless, which encodes a zinc finger transcription factor. We analyzed the function of senseless during pupal retinal development and found that senseless is required for recruitment of both cone and pigment cells, the pupal-derived ommatidial support cells. We also found that Senseless is expressed in neural precursors shortly after the larval-pupal transition and is both necessary and sufficient for interommatidial bristle development. Furthermore, senseless is the primary target of achaete and scute during interommatidial bristle development. We also identified several differences between the development of interommatidial bristles and other macrochaete. In particular, EGFR signaling is not required for interommatidial bristle development, nor is positive feedback regulation of proneural genes by senseless. A model for interommatidial bristle specification is presented.

The worm's sense of smell

Animals sense their chemical environment using multiple chemosensory neuron types, each of which exhibits characteristic response properties. The chemosensory neurons of the nematode Caenorhabditis elegans provide an excellent system in which to explore the developmental mechanisms giving rise to this functional diversity. In this review, we discuss the principles underlying the patterning, generation, differentiation, and diversification of chemosensory neuron subtypes in C. elegans. Current knowledge of the molecular mechanisms underlying each of these individual steps is derived from work in different model organisms. It is essential to describe the complete developmental pathways in each organism to determine whether functional diversification in chemosensory systems is achieved via conserved or novel mechanisms. Such a complete description may be possible in C. elegans.

HLH-14 is a C. elegans achaete-scute protein that promotes neurogenesis through asymmetric cell division

Achaete-Scute basic helix-loop-helix (bHLH) proteins promote neurogenesis during metazoan development. In this study, we characterize a C. elegans Achaete-Scute homolog, HLH-14. We find that a number of neuroblasts express HLH-14 in the C. elegans embryo, including the PVQ/HSN/PHB neuroblast, a cell that generates the PVQ interneuron, the HSN motoneuron and the PHB sensory neuron. hlh-14 mutants lack all three of these neurons. The fact that HLH-14 promotes all three classes of neuron indicates that C. elegans proneural bHLH factors may act less specifically than their fly and mammalian homologs. Furthermore, neural loss in hlh-14 mutants results from a defect in an asymmetric cell division: the PVQ/HSN/PHB neuroblast inappropriately assumes characteristics of its sister cell, the hyp7/T blast cell. We argue that bHLH proteins, which control various aspects of metazoan development, can control cell fate choices in C. elegans by regulating asymmetric cell divisions. Finally, a reduction in the function of hlh-2, which encodes the C. elegans E/Daughterless bHLH homolog, results in similar neuron loss as hlh-14 mutants and enhances the effects of partially reducing hlh-14 function. We propose that HLH-14 and HLH-2 act together to specify neuroblast lineages and promote neuronal fate.

The Drosophila proneural gene amos promotes olfactory sensillum formation and suppresses bristle formation

Proneural genes encode basic-helix-loop-helix (bHLH) transcription factors required for neural precursor specification. Recently amos was identified as a new candidate Drosophila proneural gene related to atonal. Having isolated the first specific amos loss-of-function mutations, we show definitively that amos is required to specify the precursors of two classes of olfactory sensilla. Unlike other known proneural mutations, a novel characteristic of amos loss of function is the appearance of ectopic sensory bristles in addition to loss of olfactory sensilla, owing to the inappropriate function of scute. This supports a model of inhibitory interactions between proneural genes, whereby ato-like genes (amos and ato) must suppress sensory bristle fate as well as promote alternative sense organ subtypes.

Tufted is a gain-of-function allele that promotes ectopic expression of the proneural gene amos in Drosophila

The Tufted(1) (Tft(1)) dominant mutation promotes the generation of ectopic bristles (macrochaetae) in the dorsal mesothorax of Drosophila. Here we show that Tft(1) corresponds to a gain-of-function allele of the proneural gene amos that is associated with a chromosomal aberration at 36F-37A. This causes ectopic expression of amos in large domains of the lateral-dorsal embryonic ectoderm, which results in supernumerary neurons of the PNS, and in the notum region of the third instar imaginal wing, which gives rise to the mesothoracic extra bristles. Revertants of Tft(1), which lack ectopic neurons and bristles, do not show ectopic expression of amos. One revertant is a loss-of-function allele of amos and has a recessive phenotype in the embryonic PNS. Our results suggest that both normal and ectopic Tft(1) bristles are generated following similar rules, and both are subjected to Notch-mediated lateral inhibition. The ability of Tft(1) bristles to appear close together may be due to amos having a stronger proneural capacity than that of other proneural genes like asense and scute. This ability might be related to the wild-type function of amos in promoting development of large clusters of closely spaced olfactory sensilla.

Neuronal connectivity: bis repetita placent

The mechanism that allows a sensory neuron to extend its terminal branches along the appropriate fascicle within the CNS turns out to be the same as that which positioned the fascicle earlier on, and the gene that controls this position is the same as that which determined the neuron's identity.

Drosophila tufted is a gain-of-function allele of the proneural gene amos

Tufted is a classical Drosophila mutant characterized by a large number of ectopic mechanosensory bristles on the dorsal mesothorax. Unlike other ectopic bristle mutants, Tufted is epistatic to achaete and scute, the proneural genes that normally control the development of these sensory organs. In this report, I present genetic and molecular evidence that Tufted is a gain-of-function allele of the proneural gene amos that ectopically activates mechanosensory neurogenesis. I also systematically examine the ability of the various proneural bHLH proteins to cross-activate each other and find that their ability to do so is in general relatively limited, despite their common ability to induce the formation of mechanosensory bristles. This phenomenon seems instead to be related to their shared ability to activate Asense and Senseless.